This application proposes to conduct a social epidemiological study on the emergence and spread of crack in Mexico DF. This application takes advantage of a unique set of resources: 1) The University of Houston, Center for Drug and Social Policy Research scientific and methodological expertise in successfully implementing epidemiological drug studies among hard to reach populations and 2) The National Institute of Psiquiatrma researchers who are considered Mexico's leading drug investigators. Smoking crack is a recent emerging phenomenon in Mexico. Data on treatment admissions indicate that Mexico DF admissions increased from 8% in 2003 to 40% in 2007. This international research collaboration is of importance given the potential for an emerging crack epidemic that can contribute to the rapid transmission of HIV and other health consequences. More importantly, the risk related behaviors associated with the spread of a crack epidemic could have major public health consequences with bi- national implications for Mexico and the U.S. given the highly integrated economies and large scale circular migration patterns. The application proposes to interview 150 adults who self-report crack use within the last 30 days recruited from neighborhoods in three delegations in Mexico DF. Using an ethnographic approach, we will implement an adaptive sampling methodology with elements of field-intensive outreach, Rapid Assessment for Response and Evaluation (RARE) and targeted respondent-driven sampling. Data will be collected using in-depth semi-structured ethnographic interviews and observations including elements of Ecological Momentary Assessment (EMA). Aims are to: 1) Identify and provide a detailed description of the characteristics of the targeted crack using population including socio-demographics, crack and other drug use patterns and practices and health related sexual and drug using risk behaviors;2) Describe the processes of innovation that have contributed to the emergence of crack use practices and health related high risk sexual and drug using behaviors. Focus will be on the acquisition of knowledge, satisfaction over current drugs or practices, accessibility to new emerging drug market, compatibility with existing drug subcultural norms, complexity, triability, and observability;3) Document the process of diffusion that has led to the adoption and transmission of crack use practices and related high risk sexual and drug using behaviors within the unique context of a large and densely populated city in a developing country. Focus will be on initiation, progression of use, influence of social networks, and understanding of the social and cultural context;and 4) Examine factors associated with treatment utilization and barriers to treatment as perceived by crack users. Innovations include the application of the Diffusion of Innovation Theory to the dynamics of drug epidemics in a developing country, enhancement of the existing Mexico drug surveillance system and integration of RARE and EMA into street ethnography approaches.